The present invention relates to a zoom lens. More particularly, the present invention relates to a compact zoom lens adaptable for a small-sized video camera or digital camera.
A conventional zoom lens for a camera having a high zoom ratio is generally known in the art. The lens includes a first lens group that is fixed and has a positive refracting power, a second lens group that has a negative refracting power and which is movable along an optical axis for zooming, a third lens group that is fixed and has a positive refracting power, and a fourth lens group that has a positive refracting power and which is movable along an optical axis to achieve focus correction. The first to fourth lens groups are sequentially aligned from a subject side to an image surface of the zoom lens. The zoom ratio refers to the ratio between focal length (fw) in the wide angle position and focal length (ft) in the telephoto position, that is, ft/fw. The zoom lens has a minimum focal length in the wide angle position and a maximum focal length in the telephoto position.
A small-sized camera of recent design features a compact zoom lens and a camera body of compact size. The refracting force (also known as refracting power) of each lens group of the zoom lens is raised so as to minimize the full length of the zoom lens. If, however, the refracting power of each lens group increases, the Petzval sum may increase. As a consequence, not only is optical performance of the zoom lens degraded, but eccentricity may seriously increase when assembling the zoom lens into the camera. Such a zoom lens is therefore unsuitable for a high-pixel camera.
As generally known in the art, a mirror or a prism can be aligned in the zoom lens in order to reduce the full length of the zoom lens. The mirror or prism achieves this, by bending the optical axis of the zoom lens at a right angle. For instance, Japanese Patent Unexamined Application No. 8-248318 discloses a zoom lens in which a rectangular prism is installed in a first lens group of the zoom lens so as to bend an optical axis of the zoom lens. However, the zoom lens disclosed in Japanese Patent Unexamined Application No. 8-248318 falls short in realizing compactness of the zoom lens, because the diameter and size of a first lens aligned at the front portion of the first lens group relative to the subject must be enlarged and the number of lenses forming the zoom lens must be increased.
Although demands for a compact zoom lens suitable for a small-sized video camera or digital camera gradually increase, the conventional zoom lens may not meet the demands.
There accordingly exists a need for a compact zoom lens having high performance suitable for a small-sized video camera or digital camera.